This invention relates to polymeric additives for oil compositions, more particularly, to linear polymers of styrene and hydrogenated isoprene used as viscosity index improvers.
The viscosity of lubricating oils varies with temperature. In general, oils are identified by a viscosity index which is a function of the oil kinematic viscosity at a given lower temperature and a given higher temperature. The given lower temperature and the given higher temperature have varied over the years but are fixed at any given time in an ASTM test procedure (ASTM D2270). Currently, the lower temperature specified in the test is 40.degree. C. and the higher temperature specified in the test is 100.degree. C. For two engine lubricants with the same kinematic viscosity at 100.degree. C., the one having the lower kinematic viscosity at 40.degree. C. will have the higher viscosity index. The oil with the higher viscosity index undergoes less kinematic viscosity change between the temperatures of 40.degree. C. and 100.degree. C. In general, viscosity index improvers that are added to engine oils increase the viscosity index as well as the kinematic viscosities.
The SAE Standard J300 viscosity classification system does not specify the use of viscosity index to classify multigrade oils. At one time, however, the Standard did require that certain grades meet low-temperature viscosities that were extrapolated from kinematic viscosity measurements taken at higher temperatures, for it was recognized that oils that were exceedingly viscous at low-temperatures caused engine starting difficulties in cold weather. For this reason, multigrade oils which possessed high viscosity index values were favored; these oils gave the lowest low-temperature extrapolated viscosities. Since then, ASTM has developed the cold cranking simulator (CCS), ASTM D5293, a moderately high-shear-rate viscometer which correlates with engine cranking speed at low temperatures, and today cranking viscosity limits, determined by the CCS, are defined in the SAE J300 Standard.
Today, it is also recognized that cranking viscosity is not sufficient to fully estimate a lubricant's low-temperature performance in engines. SAE J300 also requires that a pumping viscosity be determined in a low-shear-rate viscometer called the mini-rotary viscometer (MRV). This instrument can be used to measure viscosity and gel formation, the latter by the measurement of yield stress. In this test, an oil is slowly cooled over a two-day period to a specified temperature before viscosity and yield stress are determined. A yield stress observation constitutes an automatic failure in this test, while pumping viscosity must be below a specified limit to ensure that the oil will not cause an engine to experience a pumping failure during cold weather conditions. The test is commonly referred to as the TP1-MRV test, ASTM D4684.
Modern gasoline engines incorporate numerous improvements to reduce engine mechanical friction, and to make engine starting easier. These changes have largely been incorporated to improve fuel economy and reduce emissions. One result of this is that now many modern engines start at very low temperatures, and this increases the risk that engine pumping failures may occur in particularly cold weather. Pumping viscosity is a measure of the fluidity of the engine lubricant at low-temperatures. If the engine oil is too viscous to flow after an engine is started, catastrophic engine damage can occur. For this reason, it is important to develop engine lubricant components which have minimal contribution to the low-temperature, low-shear-rate viscosity in the TP1-MRV and which do not cause yield stress. Thus, it is desirable that the VI improver's contribution to pumping viscosity also be minimal.
Numerous materials are used in the formulation of fully-formulated multigraded engine oils. Besides the basestocks, which may include paraffinic, napthenic, and even synthetically-derived fluids, the polymeric VI improver, and the pour point depressants, there are numerous lubricant additives added which act as antiwear agents, antirust agents, detergents, dispersants, and pour point depressants. These lubricant additives are usually combined in diluent oil and are generally referred to as a dispersant-inhibitor package, or as a "DI" package.
Common practice in the formulation of a multigrade oil is to blend to a target kinematic viscosity and cranking viscosity, which is determined by the specified SAE grade requirements in SAE J300. The DI package and pour point depressant are combined with a VI improver oil concentrate and with one basestock, or two or more basestocks having different viscosity characteristics. For example, for an SAE 10W-30 multigrade, the concentration of the DI package and the pour point depressant might be held constant, but the amounts of HVI 100 neutral and HVI 250 neutral or HVI 300 neutral basestock might be adjusted along with the VI improver until the target viscosities are achieved. For an SAE 5W-30, it is often common practice to use only one basestock such as HVI 100 neutral with the selected VI improver concentrate, DI package, and pour point depressant.
Once a formulation has been arrived at that has the targeted kinematic viscosities and cranking viscosities, the TP1-MRV viscosity is determined. A relatively low pumping viscosity and the absense of yield stress is desirable. The use of a VI improver which contributes little to low-temperature pumping viscosity or yield stress is very desirable in the formulation of multigrade oils; it minimizes the risk of formulating an oil that may cause an engine pumping failure and it provides the user with additional flexibility in the use of other components which contribute to pumping viscosity.
When multigrade engine lubricants are manufactured, the VI improver is introduced in an oil concentrate. The VI improver is dissolved into basestock, such as HVI 100 neutral, before use. In some instances, the lubricant manufacturer may not have concentrate dissolving facilities, so the oil concentrate must be transported to the user as a concentrate rather than as a solid. To minimize transportation and storage costs, however, it is advantageous to be able to transport a VI improver oil concentrate that contains a high percentage of dissolved polymer. Thus, for two polymers that provide equivalent viscosities in multigrade oils, the one which can be handled at a higher concentration in an oil concentrate will be preferred. More polymer is dissolved in the concentrate and transportation and storage costs are minimized. It is also advantageous to dissolve the polymer in oil quickly, and at relatively low temperatures; this minimizes concentrate blend facility temperature requirements, and improves throughput.
The limitation on the amount of polymer that can be used in a VI improver concentrate is dependent on the concentrate's viscosity at storage and handling temperatures. Sufficient fluidity must be maintained so that the concentrate can be readily pumped from the storage vessel into the oil blending vessel. The amount of polymer that can be added to a concentrate to ensure handleability can be determined by measuring the low-shear rate viscosity of the oil concentrate.
Engine power losses are generally reduced when viscous friction is reduced through a reduction in an engine lubricant's HTHSR viscosity. This manifests itself in improved fuel efficiency. However, to ensure adequate engine journal bearing protection, minimum HTHSR viscosity limits are now included in SAE Standard J300, which defines the viscosity grading system for SAE grade designations. (The present version of the Standard is SAE J300, revision MAR93.)
One approach to the obtainment of improved fuel efficiency within a particular SAE grade is to formulate with a VI improver that contributes enough HTHSR viscosity so that the resultant engine oil is above the HTHSR viscosity minimum defined by SAE J300, but close to the minimum. Using such an approach, VI improvers such as those disclosed in U.S. Pat. No. 3,775,329 (St. Clair) offering a relatively high contribution to kinematic viscosity at 100.degree. C. (kinematic viscosity limits are also included in SAE J300), and a relatively low contribution to HTHSR viscosity are preferred. Two polymers representative of this prior art are given in examples 6c and 12c of this document. U.S. Pat. No. 3,772,196, which is incorporated by reference herein, discloses linear block copolymers of styrene (S) and hydrogenated polyisoprene (EP), having the structure S-EP.
In comparison to the S-EP polymers, a different balance between thickening efficiency, and HTHSR viscosity has been achieved with linear triblock copolymers of styrene (S) and hydrogenated isoprene (EP) having the block structure EP-S-EP as described in U.S. Pat. No. 4,788,361, which is incorporated by reference herein. However, the EP-S-EP copolymers provide substantially higher HTHSR viscosities than the S-EP copolymers of U.S. Pat. No. 3,775,329. For fuel economy savings, however, the S-EP polymers are preferred, provided that their HTHSR viscosity contributions are sufficient to meet SAE J300 requirements. The EP-S-EP polymers described in U.S. Pat. No. 4,788,361 are illustrated by symmetric polymers although the patent places no limitations on the relative sizes of the first and second EP blocks.